Structure and dynamics in suspensions of soft core-shell colloids in the fluid regime

摘要

We report on a detailed experimental study of the structure and short-time dynamics in fluid-regime suspensions of soft core-shell spherical particles with different molecular weights of the chains forming the soft outer shell, and therefore different degrees of particle softness, using 3D dynamic light scattering (3D-DLS). Owing to the particle softness, the liquid-crystal coexistence regime is found to be broader than that of hard-sphere (HS) suspensions. Static light scattering in the dilute regime yields form factors that can be described using a spherical core-shell model and second virial coefficients A(2) > 0 indicative of purely repulsive interactions. The particle-particle interactions are longer ranged for all considered systems except those of the smaller molecular weight chain grafted particles which show a HS-like behavior. 3D-DLS experiments in the concentrated regime up to the liquid-crystal transition provide the short-time diffusion function, D(q), in a broad range of scattering wavenumbers, q, from which the structural (cage) and short-time self-diffusion coefficients D(q(m)) and D-S = D((q qm)), respectively, are deduced as functions of the effective particle volume fraction phi = c/c*, where c* is the overlap concentration, calculated using the hydrodynamic particle radius, RH. The size of the nearest-neighbor cage of particles is characterized by 2 pi/q(m), with D(q) and the static structure factor S(q) attaining at q(m) the smallest and largest values, respectively. Experimental data of D(q(m)) and D-S are contrasted with analytic theoretical predictions based on a simplifying hydrodynamic radius model where the internal hydrodynamic structure of the core-shell particles is mapped on a single hydrodynamic radius parameter gamma = R-H/R-eff, for constant direct interactions characterized by an (effective) hard-core radius R-eff. The particle softness is reflected, in particular, in the corresponding shape of the static structure factor, while